Intermolecular Interactions
Covalent Bond Energies C-O bond 81 kcal/mol 1.43 Å C-C bond 86 kcal/mol 1.54 Å C-H bond 103 kcal/mol 1.11 Å C=C bond 143 kcal/mol 1.33 Å C=O bond 165 kcal/mol 1.21 Å Compared to most non-covalent interactions these are: • Very high energies • Very short distances • Highly dependant on orientation
Intermolecular Interactions Driving Forces for the Formation of Supramolecular Structures hydrophobic interaction <10 kcal/mol electrostatic interaction ~5 kcal/mol hydrogen bond interaction 2-10 kcal/mol p-p aromatic interaction 0-10 kcal/mol van der Waals interaction 0.1-1 kcal/mol The total intermolecular force acting between two molecules is the sum of all the forces they exert on each other.
Covalent bonding Sharing of electrons to achieve stable electron configuration Small difference in electronegativity of elements Bond energy – 50-100 kcal/mol Directional bond; between specific atoms in a specific direction, normally along the line connecting the two atoms that share a pair of electrons.
Atomic Orbitals of Carbon
d-orbitals f-orbitals
C sp3 hybridization and bond directionality Shown together (large lobes only) sp3 109.5o Hybridizing s and three p orbitals form 4 identical sp3 orbitals C
sp3 hybridization of carbon orbitals
sp2 hybridization of carbon orbitals
sp hybridization of carbon orbitals
A. Ion–Ion Interaction + - Can be a very strong bond - even stronger then covalent bonds in some cases. Can be an attractive or a repulsive force. Non-directional force Long range (1/r) Highly dependant on the dielectric constant of the medium
A. Ion–Ion Interaction Energy = (k . z1 . z2 . e2) / (.r12) k = 1 / 4πo= Coulomb constant = 9 .109 N.m2/C2 e = elementary charge = 1.6 .10-19C = dielectric constant r12 = distance between the charges The energy of an ion-ion interaction only decreases at a rate proportional to 1 / r. Therefore these are very long range forces.
A. Ion–Ion Interaction When designing a host / guest complex, what will be the energetic incentive for bringing two oppositely charged species to a distance of 3 nm of one another in water? Energy = (k . z1 . z2 . e2) / (.r12) = 9 .109 . 1 . (-1) . (1.6 .10-19)2 / 78.5 . 3 . 10-9 = -2.3 . 10 -28 / 2.4 . 10 -7 = -9.8 . 10-22 J = -0.14 kcal/mol
A. Ion–Ion Interaction 1 nm? Energy = (k . z1 . z2 . e2) / (.r12) = 9 .109 . 1 . (-1) . (1.6 .10-19)2 / 78.5 . 1 . 10-9 = -2.3 . 10 -28 / 0.8 . 10 -7 = -29.4 . 10-22 J = -0.42 kcal/mol 1 nm in Chloroform? = 9 .109 . 1 . (-1) . (1.6 .10-19)2 / 4.8 . 1 . 10-9 = -2.3 . 10 -28 / 4.8 . 10-9 = -4.79 . 10-20 J = -6.89 kcal/mol 8 % of a C-C bond
B. Ion-Dipole Interaction Non-directional forces Can be attractive or repulsive Medium range interactions (1/r2) Significantly weaker then ion-ion interactions Example: crown ether complex with alkali metal ions
B. Ion-Dipole Interaction Energy = -(k . Q . u . cosq / e . r2) Maximum when q = 0 or 180 degrees Zero when q = 90 degrees u = q . l u = dipole moment l = length of the dipole q = partial charge on dipole r = distance from charge to center of dipole Q = charge on ion
B. Ion-Dipole Interaction Example: Acetone pointing directly at Na+ ion (q = zero) at a distance of 1 nm (in chloroform) Energy = -(k . Q . u . cosq / e . r2) If q = zero = -k . Q . u / e . r2 = -9 .109 . 1.6 .10-19 . 2.9 . 3.336 .10-30 / e . r2 = -1.39 . 10-38 / 4.8 . (10-9)2 = -2.9 . 10-21 J = -0.42 kcal/mol
Intermolecular Interactions p-p interactions p – p stacking (0 – 10 kcal/mol). Weak electrostatic interaction between aromatic rings. There are two general types: face-to-face and edge-to-face: Face-to-face p-stacking interactions are responsible for the slippery feel of graphite. Similar p-stacking interactions help stabilize DNA double helix.
Intermolecular Interactions p-p interactions
Intermolecular Interactions p-p interactions Distribution of electron density in benzene molecule
Intermolecular Interactions p-p Stacking H d + d + H H d + d + - - d - H H + d + d + H H H Edge-to-face
Intermolecular Interactions p-p interactions Offset, face-to-face Face-to-face, not favorable